Chilled-Water Temperatures

Chilled water (without antifreeze) at 34°F (1.1°C) is possible with some chillers that use sophisticated evaporator-design and chiller-control methods.

System Design Options

recommends a design method that starts with condenser-water temperature difference of 12°F to 18°F [7°C to 10°C].

Standard rating temperatures

Currently, the standard rating condition temperatures in ARI 550/5905 and ARI 5609 are:

•Evaporator leaving water temperature: 44°F [6.7°C]

•Water-cooled condenser, entering water temperature: 85°F [29.4°C]

•Air-cooled condenser, entering air dry bulb: 95°F [35.0°C]

For years, these temperature definitions were the benchmarks in system designs. Today, designers apply a variety of different temperatures.

ARI 550/590 reflects this trend by allowing the chilled-water and condenser- water temperatures to be selected at non-standard points and the chiller to be tested as specified by the standard.

Currently, comfort cooling systems are designed with chilled-water supply temperatures that range from 44°F [6.7°C] to 38°F [3.3°C], and, in some cases, as low as 34°F [1.1°C]. Reasons to decrease the chilled-water temperature include the following:

•The system design more readily accommodates wider temperature differences (lower flow rates) than the standard rating conditions (see “Selecting flow rates” on page 30).

•Lower water temperature allows lower air temperatures (and flows) to be selected, resulting in reduced airside installed and operating costs.

•Colder water in the same chilled-water coil may provide better dehumidification.

•Colder water can be used to increase the capacity of an existing chilled- water distribution system. In some instances, this can save significant capital expenditures to add capacity to large central plants that have reached their flow limits.

Some system designers hesitate to use lower chilled-water temperatures, concerned that the chiller will become less efficient. As discussed in “Effect of chilled-water temperature” on page 3:

•Lower chilled-water temperature makes the chiller work harder. However, while the lower water temperature increases chiller energy consumption, it significantly reduces the chilled-water flow rate and pump energy. This combination often lowers system energy consumption.

•Lower chilled-water temperatures may require more insulation on piping to prevent unwanted condensation (“sweating”). Ensure that pipes are properly insulated at all water temperatures. Lower temperature water often does not require more insulation.

Chiller System Design and Control